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Tuesday, March 4, 2014

Soil temperature is the best indicator of when it is the right time to sow vegetables

Norfolk farmers allegedly dropped their trousers, and sat on the soil to see if it was warm enough to sow crops. A soil thermometer is probably as accurate.

Below are the Met Eireann soil temperature figures for Sherkin Island which is the closest weather station to Brown Envelope Seeds. Although soil temperatures differ a little from year to year, especially in spring and autumn, they are fairly predictable between May and September differing by only a degree or two on average. Met Eireann statistics can be found here By looking up the nearest station to you, you can see when soil temperature is suitable for the sowing of crops in your area, or you can use a soil thermometer - or you can use the Norfolk method.

Mean 10cm soil temperature for Sherkin Island

Year

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Annual

2014

7.0

6.7

7.0

6.9

2013

7.5

6.8

6.3

8.9

12.3

15.5

18.8

17.6

15.3

13.7

8.5

8.0

11.6

2012

9.0

9.0

10.0

9.7

12.6

15.2

16.1

16.8

14.8

11.5

8.4

7.7

11.7

2011

5.5

8.1

8.5

12.6

13.4

15.7

16.7

16.2

14.5

12.5

10.9

8.1

11.9

The figures for Mullingar show more variation than Sherkin which is to be expected, as Mullingar is further inland and both higher summer and lower winter temperatures occur, as the temperature of the sea buffers the climatic changes near the coast.

Mean 10cm soil temperature for Mullingar

Year

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Annual

2014

4.4

5.0

5.2

4.7

2013

4.7

4.6

4.8

8.3

12.6

16.6

20.1

18.1

15.0

12.5

7.2

6.0

10.9

2012

6.1

6.7

8.9

9.7

13.4

15.8

17.3

17.7

14.6

10.4

6.8

4.5

11.0

2011

2.7

6.0

7.1

12.3

13.0

15.5

17.4

16.3

14.4

12.1

9.2

5.1

10.9

Broad beans, rocket, kale, parsnips, peas, radishes and spinach can germinate at temperatures as low as 5ºC but may take up to a month to do so. At 10ºC a higher proportion of seeds will germinated and maximum germination of crops will be a lot faster than at 5ºC. As soil temperature reach 10ºC sometime in April or May crops such as beet, cabbage, lettuce, onions and leeks, will germinate but growth will be slow and the plants will be subject to more pressure from slugs and other leaf eating pests. At 15ºC degrees, french beans, carrots and sweetcorn will germinate, and plant growth will be a lot faster, so crops will have a better chance of competing with weeds. Soil temperatures in a polytunnel will be several degrees higher and so plants started under cover will get a head start. Corn, courgettes, pumpkins, cucumbers, tomatoes and aubergines are happier at soil temperatures of 20ºC or more and will do a lot better under cover. In a good summer they will have a chance outside and by choosing cold tolerant short season varieties chances of success are improved.

The next table shows the germination of different vegetables at different temperatures. The number in brackets is the number of days to reach the level of germination. Most seeeds will germinate between 15 and 25